The optimum output coupling coefficient (the percentage of the radiation energy in a laser cavity that is extracted as a laser beam) is a function of many of the properties of the laser. For some lasers with small gains the optimum output coupling coefficient may be as small as 1 or 2 percent, and if one tries to couple out more than say 7%, the laser will not reach threshold, and no radiation will be produced. On the other hand, lasers with very high gains like chemical lasers require output coupling coefficients like 25% or higher.
Currently energy is coupled out of laser cavities in several ways. The two most frequently used methods today, are called transmission coupling and hole coupling. For transmission coupling one of the mirrors is made by placing a partially reflecting coating on a substrate that transmits the wavelength being generated by the laser. To use transmission coupling it is necessary to produce a set of mirrors that consist of one for each percentage of the radiation that one wishes to couple out, and it is generally necessary to produce a different set of mirrors for each wavelength considered. For hole coupling one of the mirrors either has a hole cut in the center of the mirror which is generally made by placing a totally reflecting coating on a substrate that has good properties for mirror making but does not transmit the laser's wavelength, or one of the mirrors is smaller that the diameter of the optically active medium, so that radiation is coupled out around the edges of this mirror. This is the method generally used with unstable resonators. Here, again, to measure the optimum output coupling coefficient it is necessary to produce a set of mirrors with various size holes.
When full size "totally" reflecting mirrors are used to form the laser cavity (a so-called closed cavity), radiation can be coupled out by placing a beam splitter in the cavity in front of one of the mirrors. This method has the advantage of being able to change the output coupling coefficient by simply rotating the beam splitter so that the angle of incidence is changed, but this has the disadvantage of rotating the output beam through twice the angle that the beam splitter was rotated.